With these circuits you can make an insulation
tester going upto two tera ohm, hence currents will
be in pico amps, great care required in design. The
rotary switches for voltages and ganged interlocked
range switches must not be phenolic but
industrial epoxy based molded switches.

The parts list which is not in the circuit, is
listed below, the circuit is many years old, 80s.
but you may get some idea on high resistance
measurement. Use 1% MFR for all Resistors and low
leakage plastic caps for low values. These
instruments i used to calibrate with Victoreen Resitors
and a Electrometer from Princeton
Research, I faintly remember.

Theory of Operation.

The Device Under Test DUT say a transformer is
placed in a Metal tray connected to the Guard SK3
terminal. The metal tray has a 3mm glass sheet on
which DUT is kept. For 2 Tera ohm make sure
that Humidity and Dust do not affect
measurements.

Now 1000V is applied on a DUT terminal from
the high voltage supply, ( in reference to Guard SK3
at earth and 0V). Then the point where the leakage
is to be measured is connected to SK4 via a BNC
short, scope, shielded cable. The current goes
thru a shunt selected by S5 and voltage across the
shunt is measured by ICL7650 a Chopper stabilized
amplifier, with ultra low offset and bias. The
amplified output is fed to Vref of ICL7107 which
displays the Insulation Resistance.

The full range is not valid in this circuit and for
low values, change to a range where the reading is
more number of counts. The above circuit does not
include some upgrades and changes i did later using
ICL7135 etc.. If i find it i will add it later.